Maintaining the position of a nearly spherical drop of water on a hydrophobic substrate appears to be a contradiction. Typically, water droplets easily move across such hydrophobic surfaces and cannot be attached to a well-defined position.
The ability to retain a well-defined drop on a substrate may have great technological significance, including the ability to spectroscopically probe a single drop over extended periods of time and to hold a living cell for spectrocopical probing. This technique could serve as an important tool in single-molecule-spectroscopy, which is often limited by the short residence time of each molecule within the illuminated volume. It is known to immobilize molecules on a substrate in order to probe minute biological material. This is done either by directly linking the probed molecule to the substrate [1, 2, 3, 4, 5, 6] or by immobilizing the molecules by inserting them into surface-tethered lipid vesicles [7]. However, these various methods may affect the structure and/or the reactivity of the molecules.
The basic technology for fabricating arrays of silicon nanotips is known and has found numerous applications, such as field emitter arrays in vacuum microelectronic devices (e.g., flat panel displays [8]), and recently as tips for AFM microscopy [9,10]. The standard method for fabricating nanotips in silicon combines photolithography and reactive ion etching (RIE) using a silicon oxide, a silicon nitride mask, or a hydrogenated-carbon mask [8,9,11].
Many of the methods for designing new materials having large contact angles (greater than 150°) combine a set of common physical and chemical characteristics: (i) patterning roughness on two very different length scales (both μm- and nm-scales), and (ii) using chemical methods to present hydrophobic moieties at the surface[12, 13]. These super-hydrophobic surfaces (i.e. hydrophobic surface having a nano-scale roughness) have shown potential for applications in water-repelling substrates and self-cleaning surfaces (specifically silicon-based solar cells) [12, 14, 15].
It should be noted that super-hydrophobic surfaces are commonly associated with large contact angles with water drop, but also small tilting angles (i.e. roll-off), e.g. less than 5°. Notable exceptions to this observation include pinned water droplets on a film of aligned polystyrene nanotubes, [13, 16] an etched aluminum alloy composed of micro-orifices and nano-particles, [17] and a monolayer of an organo- or fluorosilane self-assembled onto a laser-ablated silicon wafer [18]. In these systems, the water droplet could be suspended upside down without rolling or falling from the surface. These works have led to a debate in the literature as to the definition of super-hydrophobicity and possible microscopic mechanism for this macroscopic observation [19, 21]. The suggested mechanism underlying this pinning effect is that the droplet wets the substrate and/his wetting creates a large contact area between the droplet and the surface. Because of the high surface roughness of the substrate, the sum total of the van der Waals forces between the droplet and the substrate integrated over this large contact area is sufficient to pin the drop onto the surface. Another hypothesis is that a placed drop on the surface creates pockets of air isolated from the atmosphere and this trapped air increases the adhesion because of a negative pressure induced by the increase in volume of an air pocket as the drop is pulled away from the surface [17, 20]. These mechanisms are similar to the one proposed to explain the capabilities of a gecko's feet [21].